Adhesives have been widely used to bond materials together, particularly to hold a variety of different materials together. There are typically two requisites for adhesives in order for them to function properly. First, an adhesive should form a strong bond to each of the materials to be joined together. Also, the adhesive should have sufficient cohesive strength so it does not come apart internally when performing its desired function of holding the two materials together. Occasionally, when bonding two different materials together it is difficult to find one adhesive material that satisfactorily adheres to each of the different materials and at the same time has sufficient cohesive strength to perform its desired function.
There are several approaches available to providing bonding between two different substrates. Usually an adhesive material with good cohesive properties can be found that strongly adheres to the first material. Subsequently, adhesion to the second material can be enhanced by using a surface treatment technique. Various types of surface treatments are available for increasing bonding properties of substrates. Typical surface treatments for enhancing adhesion include: mechanical abrasion; corona discharge; flame treatment; plasma treatment; UV radiation; chemical oxidation with the use of oxidizing agents; and the introduction of functional groups by surface chemical grafting. These treatments all modify the surface of the treated material by lowering its surface energy so as to improve its wetting characteristics. Although some of these treatment methods work well to enhance bonding of an adhesive to a substantially flat surface they are much less effective for bonding an adhesive to a rough or porous surface. Rough or porous materials have higher total surface area than flat materials, and, as a result, most of the surface is not directly available for bonding to a pressure-sensitive adhesive.
Alternatively, it is known to apply chemical surface preparation compositions (e.g., primers) to materials in order to enhance bonding properties between two materials. It is common when painting a higher energy surface, such as bare wood or clean metal, to apply a surface preparation composition in order to enhance the adhesion or wetting of the surface prior to application of the final paint layer. Similarly when applying reflective pavement marking tapes to roadway surfaces it is well known to use a chemical surface preparation composition on the roadway prior to applying a reflective pavement marking tape having a pressure-sensitive adhesive thereon.
Such chemical surface preparation compositions can be organic solvent based or water based. Typically, it is necessary to apply a surface preparation composition to a substrate, allow the solvent or water to evaporate, and then apply the adhesive-containing surface to be bonded to the substrate. The time it takes to allow for drying of the solvent or the water in a surface preparation composition can be very detrimental to the application of roadway markings where a lane or two of traffic needs to be closed for the duration of the tape application. In addition, where organic solvent-based surface preparation compositions are used, the evaporation of the organic solvent is detrimental to the environment and may exceed VOC (volatile organic content) emission standards.
Thus, there is a need for other methods of enhancing adhesion between materials, particularly two different materials.
The present invention provides methods for enhancing adhesion between at least two surfaces, preferably of at least two different materials, using a powdered surface preparation composition. Preferably, at least one of the surfaces includes a material selected from the group of metal, asphalt, concrete, ceramic, wood, masonry, glass, polymeric materials, and combinations thereof. At least one of the surfaces can optionally be a painted surface. Such a material can form a surface on which a reflective traffic control sheeting or device might be attached, such as a roadway, a bridge abutment, a traffic barricade, or a barrier. The reflective traffic control sheeting or device is typically a polymeric sheeting such as a pavement marking, a sign sheeting, or a barrier delineation sheeting.
The powdered surface preparation composition can include a powdered thermoplastic or thermoset polymer. It is preferably a crosslinked adhesive. Preferably, the crosslinked adhesive includes: at least one polymer; and at least one degradable crosslinker incorporated into the polymer. Preferably, the degradable crosslinker includes: at least one energetically labile moiety; and at least two free radically polymerizable groups, wherein the degradable crosslinker is capable of fragmentation into at least two fragments upon activation by an external energy source and wherein at least two fragments are essentially free of free radicals and ethylenic unsaturation.
The thermal spraying preferably includes flame spraying, and more preferably, reactive thermal spraying. Preferably, the powdered surface preparation composition includes a crosslinked adhesive that is at least partially decrosslinked during thermal spraying. This is an example of reactive thermal spraying.
In one embodiment, a method of enhancing adhesion between at least two surfaces includes: thermal spraying a powdered surface preparation composition onto at least a portion of a first surface; and subsequently contacting at least a portion of a second surface of a nonpowdered material with a portion of the first surface having the powdered surface preparation composition thereon.
In another embodiment, a method of enhancing adhesion between at least two surfaces includes applying a powdered surface preparation composition onto at least a portion of at least one surface, wherein the powdered composition comprises a crosslinked adhesive, and at least partially thermally decrosslinking the crosslinked adhesive composition. The powdered surface preparation composition is preferably applied by thermal spraying. Preferably, the crosslinked adhesive is at least partially thermally decrosslinked substantially simultaneously with applying the powdered composition.
In yet another embodiment, a method of enhancing adhesion between at least two surfaces includes: thermal spraying a powdered surface preparation composition onto at least a portion of at least one surface; and contacting a second surface with the surface having the powdered surface preparation composition thereon; wherein at least one surface comprises a scrim. In this way, three materials can be held together using the powdered surface preparation composition of the present inventionxe2x80x94a scrim, which is applied to a substrate, and a second material attached thereto.
Other embodiments are described in greater detail below.